接前面的例子：http://blog.csdn.net/kunshan_shenbin/archive/2008/09/03/2874375.aspx
测试代码如下：
1.package com.hb3.pack_22;<br />2.<br />3.import java.io.IOException;<br />4.import java.sql.SQLException;<br />5.import java.util.List;<br />6.7.import org.hibernate.Query;<br />8.import org.hibernate.Session;<br />9.import org.hibernate.SessionFactory;<br />10.import org.hibernate.cfg.Configuration;<br />11.12.import com.hb3.pack_22.model.User;<br />13.14.public class BusinessService {<br />15.16. public static void main(String[] args) throws IOException, SQLException {<br />17.<br />18. Configuration config = new Configuration().configure();<br />19. SessionFactory sessionFactory = config.buildSessionFactory(); <br />20. Session session = sessionFactory.openSession();<br />21.22.<br />23. String hql = "from com.hb3.pack_22.model.User";<br />24.<br />25. Query query = session.createQuery(hql);<br />26. List<?> users = query.list();<br />27.<br />28. for(int i = 0; i < users.size(); i++) {<br />29. User user = (User) users.get(i);<br />30. System.out.println(user.getName());<br />31. }<br />32.<br />33. query = session.createQuery(hql);<br />34. users = query.list();<br />35.<br />36. for(int i = 0; i < users.size(); i++) {<br />37. User user = (User) users.get(i);<br />38. System.out.println(user.getName());<br />39. }<br />40.41.42. session.close();<br />43. sessionFactory.close();<br />44. }<br />45.}
执行的结果为：
Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_chenyanshenbinHibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_chenyanshenbin
可见，没有启用Query的快取功能。
如果打算打开Query的快取功能，首先在hibernate.cfg.xml中设定hibernate.cache.use_query_cache属性：
1.<?xml version="1.0" encoding="utf-8"?> <br />2.<!DOCTYPE hibernate-configuration PUBLIC<br />3. "-//Hibernate/Hibernate Configuration DTD 3.0//EN"<br />4. "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"><br />5.<br />6.<hibernate-configuration> <br />7.8. <session-factory> <br />9. ....<br />10.11. <property name="hibernate.cache.use_query_cache">true</property> <br />12. ....<br />13.<br />14. </session-factory> <br />15.16.</hibernate-configuration>
然后在每次简历Query时，执行setCacheable(true)：
1.package com.hb3.pack_22;<br />2.<br />3.import java.io.IOException;<br />4.import java.sql.SQLException;<br />5.import java.util.List;<br />6.7.import org.hibernate.Query;<br />8.import org.hibernate.Session;<br />9.import org.hibernate.SessionFactory;<br />10.import org.hibernate.cfg.Configuration;<br />11.12.import com.hb3.pack_22.model.User;<br />13.14.public class BusinessService {<br />15.16. public static void main(String[] args) throws IOException, SQLException {<br />17.<br />18. Configuration config = new Configuration().configure();<br />19. SessionFactory sessionFactory = config.buildSessionFactory(); <br />20. Session session = sessionFactory.openSession();<br />21.22.<br />23. String hql = "from com.hb3.pack_22.model.User";<br />24.<br />25. Query query = session.createQuery(hql);<br />26. //使用Query快取 27. query.setCacheable(true);<br />28. List<?> users = query.list();<br />29.<br />30. for(int i = 0; i < users.size(); i++) {<br />31. User user = (User) users.get(i);<br />32. System.out.println(user.getName());<br />33. }<br />34.<br />35. query = session.createQuery(hql);<br />36. //使用Query快取 37. query.setCacheable(true);<br />38. users = query.list();<br />39.<br />40. for(int i = 0; i < users.size(); i++) {<br />41. User user = (User) users.get(i);<br />42. System.out.println(user.getName());<br />43. }<br />44.45.46. session.close();<br />47. sessionFactory.close();<br />48. }<br />49.}
执行结果为：
Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_chenyanshenbinchenyanshenbin
其实，Hibernate在启用Query的缓存机制后，会保留执行过的查询SQL和查询结果，在下次查询后会看看SQL是否相同，并看看对应的资料库表格是否有变动(Update/Delete/Insert),如果SQL相同且且资料库也没有变动则将缓存中的结果直接返回。
值得一提的是，Query上有list()与iterator()2个方法用于结果集返回。他们的区别是，list()方法在读取数据库时不会使用缓存机制而直接向数据库发起查询，而iterator()则会将读取到的结果写到缓存中，以便于读取时再次利用。
1.Session session = sessionFactory.openSession();<br />2.<br />3.Query query = session.createQuery("from User");<br />4.List users = query.list();<br />5.users = query.list();<br />6.7.session.close();
执行结果：
Hibernate: select user0_.id as id, user0_.name as name0_, user0_.age as age0_ from user user0_<br />Hibernate: select user0_.id as id, user0_.name as name0_, user0_.age as age0_ from user user0_
1.Session session = sessionFactory.openSession();2.3.Query query = session.createQuery(“from User”);4.Iterator users = query.iterate();5.users = query.iterate();6.7.session.close();
执行结果：
Hibernate: select user0_.id as col_0_0_ from user user0_Hibernate: select user0_.id as col_0_0_ from user user0_
由于使用iterator()方法时会使用到Session level缓存，所以在查询大量记录时会好用大量的记忆体。必要时可以使用Session的evict()或clear()方法来清除缓存。
请参阅：http://blogger.org.cn/blog/more.asp?name=NaddyLee&id=31540
请注意，尽管iterator()方法时会使用到Session level缓存,但这不能就说在任何情况下都是最有效率的做法。
请看下面的测试：
1.package com.hb3.pack_22;<br />2.<br />3.import java.io.IOException;<br />4.import java.sql.SQLException;<br />5.import java.util.Iterator;<br />6.import java.util.List;<br />7.8.import org.hibernate.Query;<br />9.import org.hibernate.Session;<br />10.import org.hibernate.SessionFactory;<br />11.import org.hibernate.cfg.Configuration;<br />12.13.import com.hb3.pack_22.model.User;<br />14.15.public class BusinessService {<br />16.17. public static void main(String[] args) throws IOException, SQLException {<br />18.<br />19. Configuration config = new Configuration().configure();<br />20. SessionFactory sessionFactory = config.buildSessionFactory(); <br />21. Session session = sessionFactory.openSession();<br />22.23.<br />24. String hql = "from com.hb3.pack_22.model.User";<br />25.<br />26. Query query = session.createQuery(hql);<br />27.<br />28.// List<?> list = query.list();29.// Iterator<?> iterator = list.iterator();30.// while (iterator.hasNext()) {31.// User user = (User)iterator.next();32.// System.out.println(user.getRoom().getAddress());33.// }34.// System.out.println("=====================================");35.// list = query.list();36.// iterator = list.iterator();37.// while (iterator.hasNext()) {38.// User user = (User)iterator.next();39.// System.out.println(user.getRoom().getAddress());40.// }41.//<br />42.// System.out.println("++++++++++++++++++++++++++++++++++++++");43.<br />44. query = session.createQuery(hql);<br />45. Iterator<?> iterator = query.iterate();<br />46. while (iterator.hasNext()) {<br />47. User user = (User)iterator.next();<br />48. System.out.println(user.getName());<br />49. }<br />50. System.out.println("=====================================");<br />51. iterator = query.iterate();<br />52. while (iterator.hasNext()) {<br />53. User user = (User)iterator.next();<br />54. System.out.println(user.getName());<br />55. }<br />56.<br />57. session.close();<br />58. sessionFactory.close();<br />59. }<br />60.}
执行结果：
Hibernate: select user0_.id as col_0_0_ from user user0_Hibernate: select user0_.id as id0_1_, user0_.name as name0_1_, user0_.room_id as room3_0_1_, room1_.id as id1_0_, room1_.address as address1_0_ from user user0_ left outer join room room1_ on user0_.room_id=room1_.id where user0_.id=?chenyanHibernate: select user0_.id as id0_1_, user0_.name as name0_1_, user0_.room_id as room3_0_1_, room1_.id as id1_0_, room1_.address as address1_0_ from user user0_ left outer join room room1_ on user0_.room_id=room1_.id where user0_.id=?shenbin=====================================Hibernate: select user0_.id as col_0_0_ from user user0_chenyanshenbin
看见，Iterator中的元素并不是一次性返回的，而是一条一条返回的。当数据量庞大时，特性就会下降。
再看下面的测试代码：
1.package com.hb3.pack_22;<br />2.<br />3.import java.io.IOException;<br />4.import java.sql.SQLException;<br />5.import java.util.Iterator;<br />6.import java.util.List;<br />7.8.import org.hibernate.Query;<br />9.import org.hibernate.Session;<br />10.import org.hibernate.SessionFactory;<br />11.import org.hibernate.cfg.Configuration;<br />12.13.import com.hb3.pack_22.model.User;<br />14.15.public class BusinessService {<br />16.17. public static void main(String[] args) throws IOException, SQLException {<br />18.<br />19. Configuration config = new Configuration().configure();<br />20. SessionFactory sessionFactory = config.buildSessionFactory(); <br />21. Session session = sessionFactory.openSession();<br />22.23.<br />24. String hql = "from com.hb3.pack_22.model.User";<br />25.<br />26. Query query = session.createQuery(hql);<br />27.<br />28. List<?> list = query.list();<br />29. Iterator<?> iterator = list.iterator();<br />30. while (iterator.hasNext()) {<br />31. User user = (User)iterator.next();<br />32. System.out.println(user.getRoom().getAddress());<br />33. }<br />34.// System.out.println("=====================================");35.// list = query.list();36.// iterator = list.iterator(); <br />37.// while (iterator.hasNext()) {38.// User user = (User)iterator.next();39.// System.out.println(user.getRoom().getAddress());40.// } <br />41.//42.// System.out.println("++++++++++++++++++++++++++++++++++++++");43.<br />44. query = session.createQuery(hql);<br />45.// Iterator<?> iterator = query.iterate();46.// while (iterator.hasNext()) {47.// User user = (User)iterator.next(); <br />48.// System.out.println(user.getName());49.// }50. System.out.println("=====================================");<br />51. iterator = query.iterate();<br />52. while (iterator.hasNext()) {<br />53. User user = (User)iterator.next();<br />54. System.out.println(user.getName());<br />55. }<br />56.<br />57. session.close();<br />58. sessionFactory.close();<br />59. }<br />60.}
执行的结果为：
Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_Hibernate: select room0_.id as id1_0_, room0_.address as address1_0_ from room room0_ where room0_.id=?NTU-M8-419NTU-M8-419=====================================Hibernate: select user0_.id as col_0_0_ from user user0_chenyanshenbin
这明显在性能上要优于刚才的做法。可见，首次查询使用query.list性能要优于query.iterate，之后再做相同的查询请求时，则借助缓存机制，使用query.iterate会大大提高性能。
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